Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups. Part 2 / 0747 821

16. Photochemistry of nitro and nitroso compounds

767

o-Nitrobenzyl photorearrangements have been applied to the area of photocatalysis, microlithography and biosensors. For example, the photolytic generation of acid has been developed as a potential candidate for radiation-sensitive materials for microelectronics and coating industry48. The systems of photogenerated acids from o-nitrobenzyl carboxylates49 and sulphonates50 have been applied as novel photoactive resists. The acid photogenerators based on the 2-nitrobenzyl rearrangement have been reviewed51.

Explorative studies of applying o-nitrobenzyl photochemistry to generate amines and diamines were reported, that is, to use the o-nitrobenzyloxy group as a masking group which can be photolytically detached52. The quantum efficiencies of the photodecomposition of o-nitrobenzyl carbamates 62 and 64 (equation 40) have been studied in solution and in the solid state52. The 2,6-dinitrobenzyl carbamates undergo photodecomposition most efficiently with quantum yields as high as 0.62 for 66, R1 and R2 D cyclohexyl; the photosensitivites are controlled by a complex combination of both steric and electronic effects.

X		X
R	O
CHO	CNR1R2	hν	+ HNR1R2
		COR + CO2

(40)

NO2			NO2
(62)	R = H, X = H	(63)	R = H, X = H
(64)	R = alkyl, X = H	(65)	R = alkyl, X = H
(66)	R = alkyl, X = NO2	(67) R = alkyl, X = NO2

Time-resolved resonance Raman spectroscopy has been used to study the photorearrangement of o-nitrobenzyl esters in polar and protic solvents53; in acetonitrile, the only primary photoproduct is nitronic acid 68 with a lifetime of 80 microsecond, while in methanol the nitronic acid exists in equilibrium with the nitronate anion 69, giving a lifetime of 100 microseconds (equation 41).

	O			O
CHROCR′			R	CR′
			CO
	hν
NO2			+	OH
			N
		(68)	O−	(41)
	O			R
	R
	COCR′			O
		+ H +		O
	+ O−			+ R′ C
				OH
	N			NO
(69)	O−

Photochemistry of (2-nitrophenyl)diazomethane 70 has been studied by excitation at 350 nm in argon matrix isolation system54. That shows that at 10 K, 2-nitroso- benzaldehyde is formed by intramolecular oxygen migration from (2-nitrophenyl) carbene

768	Tong-Ing Ho and Yuan L. Chow

71. Further irradiation ( > 350 nm) of 2-nitrosobenzaldehyde causes secondary reactions to give a mixture of 2,1-benzisoxazol-3(1H)-one 72 and carbonylcyclopentadiene imine 73 along with carbon dioxide (equation 42). It was shown that oxazolone 72 undergoes decarboxylation to give 73 upon photolysis with shorter-wavelength light ( > 300 nm) but not with longer-wavelength light ( > 350 nm). Irradiation ( > 350 nm) of (4-n- butyl-2-nitrophenyl)diazomethane (74) in argon matrix at 10 K results in the formation of the oxazolone 77 and imine 78 that may be derived from intermediate 76 upon further irradiation (equation 43). Photoexcited nitrosobenzaldehyde 75 must undergo H- atom transfer to give intermediate 76, which could spontaneously cyclize from either diradicaloid or ketenoid forms to give oxazolone 77. However, it requires deep-seated rearrangements and corresponding energy to reach the ketimine stage; the nitrene and carbene species have been proposed to mediate the changes.

CHN2				CH		CHO
	A r2
	hν
NO2				NO2		NO
(70)		(71)		hν
							(42)
	O			(>350 nm)
	O	hν			NH + CO2
				C
	N	>300 nm
	H
(72)				(73)
	CHN2				CHO
		A r, 10 K
		>350 nm
n-Bu	NO2		n-Bu		NO
(74)				(75)
	O
	C			>350 nm
	OH						(43)
					O
n-Bu	N
(76)		hν				+	C NH
					O
	O	n-Bu			N	n-Bu
					H
	C						(78)
				(77)
	OH
n-Bu	N

16. Photochemistry of nitro and nitroso compounds	769
The application of o-nitrophenylethylene glycol as a photolabile protective	group

of aldehydes and ketones was further discussed55. The deprotection of 1,3-dioxolane

group can	be carried	out	by	photolysis at	350 nm	in	an inert solvent	such
as benzene,	giving fair	to	high	yields. The	isolation	and	characterization	of o-

nitroso-˛-hydroxyacetophenone demonstrates a mechanistic link with the known photorearrangement of o-nitrobenzaldehyde to o-nitrosobenzoic acid. The scope and limitation are also discussed mainly on the basis of its stability to bases and acids (equation 44).

NO2

hν

H

O

O

R R

O −

N

+ OH

O

O

R R

O

+

R R

O −

N

+ O

H

O

O

R R

OH

N

O

O O

R R

NO

OH

O

O −

N

+ OH

O

O

R R

O

R

O

R

O

N

OH

(44)

O

R

NO

OH

O

R

A series of o-nitrobenzyl derivatives derived from glutamine, asparagine, glycinamide and -aminobutyramide linked through the amide nitrogen are photolysed to release free amides according to the common mechanism shown in equation 4656. The quantum yield for the release of glutamine (equation 45) from the ˛-methyl derivative is 0.13 and that from the carboxy derivative is 0.24. The mechanism involved the aci-nitro anion intermediate 80 (equation 46); the half-lives for the ˛-methyl, ˛-carboxyl and ˛-H derivatives of glutamine at pH 7.5 are 360, 720 and 1800 microseconds respectively.

770	Tong-Ing Ho and Yuan L. Chow
	NO2	NO
	hν
	NHCO(CH2 )2 CHNH2		O
	CO2 H	R		(45)
	R
	+	H2 NCO(CH2 )2 CHNH2
			CO2 H
	R = H, CH3 , CO2 H
		O−

NO2

H		hν
	NHCOR′
R
O−
N
+	O−	+ H+

NHCOR′

R

(80)

N

+ OH

NHCOR′

R

(79)

NO

(46)

OH

NHCOR′

R

NO2

+ H2 NCOR′

O

R

c. Nitrobenzenes with ortho CDX bonds and their derivatives. Mechanistic studies on the photochemistry of o-nitrobenzaldehyde by the matrix-isolation technique have provided evidence for the existence of a ketene intermediate57 on excitation at 313 or 350 nm (equation 47); the ketene is the precursor of the o-nitrosobenzoic acid and of the N- hydroxybenzisoxazolone. Excitation at 357 nm leads to the exclusive formation of the former acid.

16. Photochemistry of nitro and nitroso compounds					771
O				O
C				C
	H
		> 357 nm		OH
NO2				NO
313 nm					(47)
350 nm			313 nm, 350 nm	O
	O			C
C

	313 nm, 350 nm	O
		N
NO2 H		OH

This photoreaction has been investigated by laser flash photolysis58 and quantum yield measurements that identify the triplet state ( D 6 nanoseconds) as the reactive species, and show intermediate 82 is sensitive to hydroxylic molecules, but the logical precursor biradical intermediate 81 could not be detected owing to a short lifetime (equation 48).

O		O		O
C				C
Η
hν
+ O		+	OH	+ OH
				N
N		N
O−		O−		O−
	(81)		(82)	(48)
			H2 O	OH
CO2 H				+ O
				N
OH			CO2 H	O−
N

OH

NO

Application of similar photochemistry in the carbohydrate domain has been reported by Collins and coworkers who demonstrated that O-(2-nitrobenzylidene) sugars 83 and 85 can be sequentially decoupled by photolysis and oxidation to give hydroxy-O-2-nitrobenzoyl

derivatives (84 and 86) that are specifically partially protected sugars59	(equation	49).
In both cases, the first-stage photolysis specifically deprotects the C-3	equatorial	OH

group. Such a blocking deblocking sequence including the key-photoreaction step is

772	Tong-Ing Ho and Yuan L. Chow
applied to transform	87 to 88, during which the C-3 equatorial OH group is bared

for the specific glucosylation60 to afford 89 (equation 50); this corresponds to the fully esterified methyl-3,4-di-O-(ˇ-O-glucopyranosyl)-˛-L-rhamnopyranoside (90, not shown). In a similar fashion, the fully protected ˇ-galactosyl-˛-galactoside derivative 91 is photolysed and oxidized to give the corresponding intermediate of 92 carrying the C-3 OH group61.

			OMe			OAc
Me		O	O	AcO	O	R
							hν
		O				AcO
	O							OMe
						Me	O	AcO
		Ar						O
							OH
					NO2		OCAc
							O
(83)	R = α -OAc		Ar =				(84)	R = α -OAc
(85)	R = β -OAc						(86)	R = β -OAc
		OAc				OMe
AcO			O	Me		O
				O
AcO		OAc
					O	O	hν
			(87)		H
						Ar
							OAc
						AcO	O	Me
								O
						AcO	OAc
							OMe	(88)
		OAc
			O	Me		O
AcO
				O
AcO			OAc			O
				AcO	O	OCAr

O

OAc

OAc OAc

(89)

NO2

	OAc
O	R (49)
	AcO

OMe

O

HO

OCAr

O

(50)

Ar =

	16. Photochemistry of nitro and nitroso compounds						773
O				OCOAr
	OCOCMe3			OCOCMe3
Ar		O			O
O				HO
	O	O	(CH2 )3 CO2 Me	O	(CH2 )3 CO2 Me
					O
		OAc	OAc		OAc	OAc
			AcO			AcO
		O			O
			AcO			AcO
		(91)			(92)

d. Nitrobenzenes with ortho heteroatom substituents and their derivatives. The photocyclization of N-acyl-2-nitrodiphenylamines is an efficient reaction to give phenazine N-oxides as shown62 in equation 51. Irradiation of compound 97 gives N-oxides 98 and 99 in equal amounts (equation 52). When compound 95 is photolysed in the presence of trifluoroacetic acid, the acetyl group is eliminated efficiently to give 100 predominantly (equation 53); the acyl group must end up as the mixed anhydride. The overall reaction pattern involving equations 51 53 is summarized in Scheme 5, in which the key intermediate 103 can be trapped by 2,6-di-tert-butylphenol (DTBP) and triphenyl phosphine (TPP) to afford 104 and 105, respectively.

	Ac
	N			N
		hν
X	NO2	H		N	X
					(51)
				O
(93)	X = H		(94)	X = H (80%)
(95)	X = NO2		(96)	X = NO2 (98%)
	Ac
	N	NO2	N	X
		hν
	NO2		N		(52)
			O	Y
	(97)	(98)	X = NO2 , Y = H
		(99)	X = H, Y = NO2

774		Tong-Ing Ho and Yuan L. Chow
		Ph
		N	Me
			O
	Ph	NO2
					Ph
	N	Me			N	Me
		O				O
	O
						O
	N
					N
	O
	(101)				O	H
	TFA				(102)
						N
	Ph	N				Ph
	N		Ph	D TBP
	H					NO
			O	Me
						(104)
		N
						H
	NO2		O		TPP
		O				N
		(103)				Ph
	N			N		N PPH3
				+	Ph	(105)
	N			NO
			(106)
	O
		SCHEME 5
			H
			N
95	hν	+
	96 (13%)					(53)
	CF3 CO2 H
		O2 N	NO2

(100) 85%

Excitation of o-nitrophenyl alkyl ethers (107 and 108) causes the intramolecular hydrogen abstraction from the n Ł triplet state of the nitro group to give benzoxazoles 109 and 110 respectively63 according to the mechanism in equation 54.

	16. Photochemistry of nitro and nitroso compounds					775
			H
	OCH2 R		O
			R		O
	hν					H
	NO2		N OH		N	R
						OH
(107)	R = Ph		O		O
(108)	R = Me					(54)
	O			O
		R	hν	R
	N			N
	O		(109)	R = Ph (62%)
			(110)	R = Me (61%)

Excitation of o-nitrodiphenylamine (111) and 3-nitro-2-phenylaminopyridine (112)64 causes the common intramolecular hydrogen abstraction, as the initial step, but subsequent steps involve the elimination of HNO2 and cyclization to give carbazole and ˛-carboline (113 and 114) in equation 55.

			OH
	NO2		N
		hν	O
X	N	X	N
	H

(111)X = CH

(112)X = N

	+ OH			H
	N
	O	−HNO2		(55)
X	N		X	N
	−

XN H

(113)X = CH

(114)X = N

776	Tong-Ing Ho and Yuan L. Chow

Irradiation of 1-alkoxy-4-t-butyl-2,6-dinitrobenzenes apparently gives aniline the aniline intermediates that undergo acyl migration to give the more stable anilide (equation 56)65.

	OCH2 R		OCOR		OH
O2 N	NO2	O2 N	NH2	O2 N	NHCOR
	hν

(56)

R= H, CH3 , C2 H5

2.Nitro nitrite rearrangement

Derivatives of 9-nitroanthracens 115 undergo the nitro nitrite rearrangement from their triplet n, Ł state to 9-anthrol derivatives 118 as shown66 in equation 57; nitrite photolysis is well known and ESR spectra for the anthryloxy radical 117 can be recorded at room temperature.

O

N

NO2

O

hν

X	X
(115) X = CN, C6 H5CO	(116)	(57)
O		OH

+ NO

X	X
(117)	(118) X = CN, C6 H5CO

D. Photoaddition and Coupling

Irradiation of a mixture of styrene and a nitroarene, such as nitrobenzene, 2- nitrothiophene or 2-nitrofuran, in acetonitrile gives the corresponding nitrone in high